The present invention relates generally to the field of metal-elastomer composites and the problem of reinforcement of polymers, elastomers and especially rubbers with metal elements such as elongated steel elements. More particularly the present invention relates to a composite of a vulcanizable rubber or rubber-like composition with one or more metal reinforcement elements embedded therein, and a cured rubber or rubber-like product obtained by vulcanization of said composite.
Among the known composites of metal and elastomers, those wherein reinforcing elements of steel or steel alloys are bonded to rubber find a most wide application in a variety of products including conveyor belts, heavy duty hoses, pulley belts and pneumatic tires.
In today""s tire technology, metal reinforcements are coated with a brass layer that undergoes a chemical reaction during curing with a sulfur curable rubber composition, leading to a surface layer predominantly consisting of a copper sulfide layer. This copper sulfide layer facilitates adhesion by mechanical and/or chemical interaction with the cured rubber. Rubber compositions of adhesion rubbers in today""s technology have to be fine tuned to support the very specific chemical requirements of adhesion layer formation, and the curing system has to be adjusted to the delicate kinetics of copper sulfide formation in order to reach the optimum balance between size and structure of the copper sulfide dendrons. Rubber compositions that are fine tuned for a variety of uses, such as skim compounds in tires, are high cost materials due to their necessity for a large amount of insoluble sulfur and expensive adhesion promoters like cobalt containing chemicals. Furthermore, compounding for adhesion rubbers is currently a compromise between the demands of the adhesion system and other demands like the resistance against aging phenomena, corrosion, fatigue and durability under the high shear forces acting in tires that are in service.
Numerous adhesion systems have been proposed, where organic adhesives are interposed between the metal and the elastomer or plastic material to create an improved metal-to-elastomer/plastic bond. WO 97/17144 A1, for example, teaches a composition suitable for treating metal surfaces prior to bonding them to other materials, including other metals, rubber, glass, polymers, sealants, and coatings, to enhance bond strength and to prolong useful life in corrosive environments. The composition comprising an organoalkoxysilane is capable of crosslinking when applied to the metal surface to form an adherent coating and is also capable of bonding, via functional groups, with the material to be bonded to the metal surface to form a strong adherent bond.
In particular, for materials under high stress in service, the use of a thin organic adhesive layer alone may not suffice because there is still the problem of adhesion loss caused by high dynamic loads and shear forces acting across the interface between the rigid surface of the metal with the adhesive and the polymer matrix.
Patent abstracts of Japan vol. 8, no 37, JP 58193134A disclose the process of coating a steel cord with liquid rubber, bearing COOH or OH functional groups, and vulcanizing said coated cord embedded in a type of rubber. According to this process, the steel cord is first transferred into a liquid rubber bath containing preferably a liquid polyisoprene rubber with an average viscosity molecular weight of 15000-50000. Although the liquid rubber is of comparatively low molecular weight, the viscosity of the liquid rubber bath is too high to create thin coating layers. This leads to weak boundary layers with the embedding rubber.
FR 2320974 A discloses a coating for a steel reinforcement element of a first layer of an organosilane and a second layer of an organic RFL adhesive, i.e. an aqueous emulsion of a resorcinol-formaldehyd resin and a rubber-containing latex, which is cured thereafter. This product can be used as a reinforcement element for rubber products. However, the curing of the RFL adhesive makes it impossible to get a smooth boundary layer between the coating and the rubber. Moreover, the non-reactive rubber-latex is only held mechanically within the structure of the crosslinked resin and cannot contribute to the bonding between the coating layer and the rubber to be reinforced.
FR 2271036 A discloses a coating for a steel reinforcement element of a first layer of a vinyl polymer comprising OH- and/or COOH-groups and a second layer of a composition of an organic RFL adhesive and a styrene-butadiene vinyl pyridine terpolymer, where both layers are finally heat treated. The same drawbacks as described for FR 2320974 A arise for the coating disclosed in Fr 2271036.
It is therefore an object of the invention to provide an adhesion promoting system between a metal surface and an elastomer that allows for superior resistance against aging, corrosion, dynamic loads and shear forces acting across the interface.
The aforementioned object of the invention is achieved by a special adhesion promoting coating of the metal, i.e. by a composite of a vulcanizable rubber or rubber-type composition with one or more metal reinforcement elements embedded therein, the metal reinforcement element being coated with
a polymer or non-cured rubber composition deposited from an aqueous, alcoholic or organic solution and compatible with and co-polymerizable, co-vulcanizable or crosslinkable with said vulcanizable rubber or rubber-type composition to be reinforced, and additionally bearing functional groups
either covalently bonding to the metal surface of said reinforcement element
or forming covalent bonds with the outward directed first functional groups of a mono- or multimolecular layer of a bifunctional adhesion promoter intercalated between said metal and said coating and bound to said metal by its second functional groups;
as well as a cured rubber or rubber-like product obtained by vulcanization of the above composite. This product can be, for example, a pneumatic tire, a hose or a belt, such as a conveyor belt or a pulley belt.